Tuesday, February 15, 2011

A few people have used a couple layers of ordinary window screen as the absorber for solar air heating collectors. This is the same stuff you put in windows to keep the bugs out.
Scott and I have been doing some testing of air heating collectors of various types, and to the surprise of some, the collector that uses the two layers of window screen as the absorber has done as well or better (so far) as other more complex absorbers.

The way these collectors work is shown in the diagram -- air enters the collector via the bottom vent and is routed to the glazing side of the screen absorber. The screen has a little bit of pressure drop and this tends to spread the air evenly over the screen. As the air works its way through the screen, it is heated, and eventually makes it way out the top vent located at the top back of the collector behind the screen.

In an alternative to this picture, it has been suggested by Laren over at the Yahoo SimplySolar group that the screen is really quite open, and that much of the sun passes right through the screen and ends up getting absorbed instead by the back wall of the collector. In this view, the screen still helps, in that it does absorb some of the solar, but, perhaps more importantly, it keeps the heated air from migrating back to the cold glazing where heat losses would be high.

I did a little test to measure how much of the incident sunlight 1, 2 and 3 layers of fiberglass and aluminum screen absorb, and some tentative conclusions and some questions come out of this. So, take a minute to look at the results and see if you have any ideas to contribute on making these absorbers more efficient -- or, maybe you think there is a better option altogether?

15 comments:

hi Gary ... good idea using screen ... there is a design by Bill Kreamer that uses black polyester felt ... I really like the screen idea as it is probably cheaper than felt... I would suggest using more layers of screen possibly five or six and making the box interior reflective rather than absorbive (if thats a word) ... the key I think is to have only the excahnger absorb heat and to make sure air passes thru the material in a turbulent manner therefore increasing heat exchange ... as for glazing it appears double glazing is best but a single glaze is still good.

Hi -- There is some testing on absorbtion vs number of layers, and it indicates 2 or 3 layers do a good job. The test details are one of the links under the "Experimental" tab on my site.

On on the fence about reflective or absorbtive back -- heat absorbed by a black back wall is still absorbed by the air (less a little that leaks through the back insulation) but the back may run at a pretty high temperature to accomplish this, which means more radiation toward the glazing, but this radiation has to get through a couple screen layers to get out. Also think that a reflective back would work well -- maybe with an added layer of screen to let less light get reflected back out the glazing.

I thought the problem with screen-based collectors was the warm and humid indoor air would condense on the colder exterior surface quickly making the interior of the glazing full of condensation and in time very dirty. What is your experience in cold climates such as Wisconsin?

Hi Mike,I would like to try the felt absorber -- don't think I can duplicate Bill's interior geometry and still have something I can compare to other collectors, but it would be nice to see how well the felt does.

A test with the geometry of your solar air heating collector is what I was thinking of. Testing felt should be a straight forward change out with the window screen. Reflective vs flat black interior could be as simple as tacking tinfoil to the inside surfaces of the box.

I'm assuming the aluminum screen was natural and not painted black. How much difference would a black coating make? I saw a post where the fiberglass outgassed. Does that make it a no go or is it something that is fixable? I've seen shadecloth from greenhouse companies that go from 30% shade to 90%shade. The material is polypropylene I believe. Do you think that might make a suitable screen?

Hi John,I use the aluminum screen that is sold as "charcoal" in color -- its very near black. The black appears to be something like an anodize, and does not appear to have any problem with outgassing.I did have some problems with the fiberglass screen outgassing and leaving a kind of gooey liquid on the glazing:http://www.builditsolar.com/Experimental/ScreenAbsorber/ScreenAbsorber2.htmI'd not recommend use the fiberglass screen for an absorber.

Thanks for the quick reply! I am considering putting some solar collectors on my house. The way it makes sense to me would be to have a very wide one on the south side vertical wall of the house. If the collector was to be 25' or so wide, would the screen type have too much pressure drop? Would the downspout type be a better suit for it? Has the downspout type been compared to the screen in terms of efficiency?

Hi John,The screen type pressure drop for that large a collector will be larger than for a small one, but its probably as low as you are going to get for a collector of that size. Probably the biggest problem for a large collector like that is getting an even flow to the whole screen or downspout area. I think that an air distribution plenum across the whole inlet edge, and a similar air pickup plenum across the whole return edge coupled with the screens own tendency to spread the airflow evenly would probably do the job.

It might be easier to get an even airflow with the downspouts, but you would still need carefully done plenums to distribute the flow evenly to the downspouts.

Just to give an idea how low the pressure drop is for screen collectors, this is the one I use to heat my outbuilding shop:http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htmIt has no fan at all -- the bouyancy of the heated air is enough to move air through the collector.

I am going to use the screen that is labeled "solar" which claims to block more sun (to provide a cooler home), but it blocks the sun by absorbing it better, because it has more fibers per square inch. A regular screen is a web of single strands, but the solar screen uses more, for example 2 strands x 2 strands or 3 x 3, etc., so more strands to get heated by the sun. I am also building my heat collector with a built-in pre heater on the input side.